Highly corrosion-resistant fixed blade assembly for a steam turbine, in particular a geothermal impulse turbine

ABSTRACT

A highly corrosion-resistant fixed blade assembly ( 20 ) for a steam turbine, in particular a geothermal impulse turbine, is formed by an array ( 11 ) of stator blades ( 12 ), supported at their respective longitudinally opposite ends ( 27, 28 ) by an outer ( 21 ) and an inner ring ( 22 ). Each blade ( 12 ), made with a nickel-based metal alloy, for example, an alloy of the Hastelloy® family or a similar material, is integrally connected to the rings ( 21, 22 ) and is supported by the rings via respective mechanical joints ( 31, 32 ) devoid of structural welds, so as to be removable from the rings without breaking the rings.

TECHNICAL FIELD

The present invention concerns a highly corrosion-resistant fixed bladeassembly for a steam turbine, in particular a geothermal impulseturbine; the invention also concerns a steam turbine, in particular ageothermal impulse turbine, equipped with this fixed blade assembly.

BACKGROUND ART

As is known, in steam turbines the transfer of energy from the steam tothe shaft takes place through a successive series of expansions of thesteam that, expanding in the nozzles that constitute the fixed part ofthe turbine, increases in speed, therefore transforming the steam's heatenergy into kinetic energy; the kinetic energy is transmitted to theshaft through mobile blades mounted on the circumference of the shaftitself.

In order to use the energy within the steam in an efficient manner, asteam turbine is normally composed of a number of successive stages,which for axial-flow turbines are arranged coaxially with the axis ofrotation of the mobile blades (machine axis), so that the steamdischarged from one stage flows directly and more or less axially intothe next.

In impulse turbines, each stage includes a mobile blade assembly, formedby an array of blades carried integrally by the shaft, and a fixed bladeassembly, usually called a diaphragm, facing the mobile blade assemblyand formed by an array of fixed blades, opportunely shaped and arrangedradially with respect to the machine axis. The fixed blades aresupported by a pair of rings, radially internal and radially externalrespectively and normally composed of a pair of coupled half-rings; eachblade is welded at both ends to the support rings.

It is also known that steam turbines find one of their possibleapplications in geothermal power generation plants. In these plants, thefluid that evolves in the turbine consists of endogenous steam ofnatural origin, i.e. steam generated directly in the earth, instead ofsteam produced by conventional boilers fed with fossil or nuclear fuels,or by heat-recovery boilers as in combination-cycle systems.

With respect to boiler-generated steam, endogenous steam ischaracterized not only by much lower than normal thermodynamicconditions (substantially pressure and temperature), but also byuncontrolled chemistry which depends strongly on the site from which thesteam is extracted.

Therefore, unlike other types of systems, in which dedicated systems forprocessing the condensate used to produce the steam ensure optimum steamcharacteristics, the steam supplied to the turbine in geothermal systemsis normally less than optimum, especially as regards chemicalaggression. In addition to noncondensable gases, endogenous steam infact normally contains various substances in various forms which impartaggressive characteristics to it which are totally absent in industrialsteam, and which result in the formation of deposits on the steamturbine blades.

These effects are particularly noticeable in the stages in which thesteam expanding in the turbine passes from the superheated state to thesaturated state, and results in severe corrosion/erosion on the fixedblades and on the mobile blades.

The blades (fixed and mobile) are normally made of martensitic stainlesssteels, for example, the AISI 403 type or similar. These materials are agood compromise between mechanical characteristics and erosionresistance characteristics, and are therefore widely used in steamturbines, even in geothermal systems.

Nevertheless, these materials have the problem of poor resistance tocorrosion and stress corrosion, particularly in the presence ofchloride, as in the case of geothermal system steam turbines, andparticularly at the stages where the steam passes from the superheatedto the wet state, where the condensation mechanism enriches theconcentration of impurities in the steam.

It follows, in particular, that the service life for the fixed blades isrelatively short, especially for the blades in the turbine stages inwhich the change of phase takes place, with consequently high operatingcosts related to the need to proceed with substitution of the bladesrelatively frequently; clearly, the plant stoppage times for carryingout the maintenance operations also generate significant costs due tolost production.

Secondly, since, as previously pointed out, the fixed blades in impulseturbines are welded at both of their ends to support rings (orhalf-rings), in the case of deterioration of a blade, it becomesnecessary to substitute the entire diaphragm. In fact, in the knownsolutions, any mechanical joint systems for the ends of the fixed bladesto support rings only serve for the positioning or centring of theblades, but do not have a structural function and are intrinsicallyunable to support the blades independently, always requiring structuralwelding to ensure the fixing of the blades.

DISCLOSURE OF INVENTION

One object of the present invention is that of providing a highlycorrosion-resistant fixed blade assembly for a steam turbine, inparticular a geothermal impulse turbine, which is devoid of theabove-described drawbacks of known art.

The present invention therefore concerns a highly corrosion-resistantfixed blade assembly for a steam turbine, in particular a geothermalimpulse turbine, as essentially defined in the attached claim 1 and, forits preferred aspects, in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described further by way of example in the followingnon-limiting embodiment, with reference to the attached FIGURE, which isa partial schematic view in longitudinal cross-section of a geothermalsteam turbine equipped with fixed blade assembly in accordance with theinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the FIGURE, a steam turbine, in itself generally known and thereforeonly schematically and partially represented, is designated by referencenumeral 1. In particular, the turbine is an axial impulse turbine, usedin a geothermal power generation plant. The direction of the steam inthe turbine 1 is schematically indicated by arrow 3.

In general terms, the turbine 1 includes a casing 2 containing a stator5, integrally carried by the casing 2, and a rotor disc 6, integrallyconnected to a drive shaft 7 running through the casing 2 along arotation axis A; as usual, the casing 2 houses a number of successivestages 10. Each stage 10 is defined by an array 11 of fixed statorblades 12, integral with the casing 2 and projecting from an internalwall 13 of the casing 2 in a substantially radial direction with respectto axis A, and an array 14 of mobile rotor blades 15, carried by theshaft 7 and integrally rotating with the shaft 7.

Each stage 10 includes a fixed blade assembly 20, constituting adiaphragm of the stage 10 and formed by blades 12 and by two supportrings 21 and 22, respectively externally radial and internally radial,substantially coaxial around the axis A and preferably formed byrespective pairs of half-rings 23 and 24 coupled close together; theblades 12, having substantially known geometry and profile, aresubstantially arranged in a radial pattern around the axis A andradially extend inside the wall 13 of the casing 2 towards the shaft 7between the rings 21 and 22.

The outer ring 21 (formed by half-rings 23) is fixed, in a known manner,to the wall 13; the inner ring 22 (formed by half-rings 24) carries asealing group 25, of known type, facing the shaft 7.

Each blade 12 extends along a longitudinal axis L between twolongitudinally opposite ends 27 and 28 provided with respectiveconnection portions 29 and 30 that project transversally from the ends27 and 28 of the blade 12; the connection portions 29 and 30 are made asan integral part of the blades 12 in the same material as the blades 12.

Each blade 12 is integrally connected to the rings 21 and 22 (andspecifically to a half-ring 23 and a half-ring 24) and is supported bythe rings 21 and 22 via mechanical joints 31 and 32 devoid of structuralwelds, so as to be removable from the rings 21 and 22 (or from thehalf-rings 23 and 24) without breaking the rings 21 and 22. The joints31 and 32 have undercuts 33 that longitudinally constrain the blade 12to the rings 21 and 22, along the longitudinal axis L of the blade 12.

The term “structural welds” is intended to indicate welds capable ofensuring firm connection of components under normal operatingconditions, and specifically capable of ensuring that the rings 21 and22 effectively support the blades 12 in use.

In particular, each blade 12 is integrally connected to the half-rings23 and 24 and is supported by the half-rings 23 and and thus by therings 21 and 22 via mechanical joint members 35 arranged at the ends 27and 28 of the blade 12; each blade 12 is supported by the rings 21 and22 exclusively via the joint members 35, and not via welding. Thestructural support function of the blades 12 is exclusively carried outby the joints 31 and 32; in other terms, the blades 12 do not form amonolithic body with the half-rings 23 and 24, but are instead separablefrom the half-rings 23 and 24.

The joint members 35 of each blade 12 comprise a pair of male elements36 that engage with respective female elements 37 having a shapematching the shape of the male elements 36. The male elements 36 areinserted in the respective female elements 37 with clearances that areeliminated by wedges or shims (known and not shown for simplicity); thefinal coupling between the male elements 36 and the respective femaleelements 37 is therefore devoid of play (zero-clearance). Each blade 12carries a male element 36 and a female element 37 arranged at the ends27 and 28 of the blade 12.

In the shown example, the connection portions 29 facing towards theouter ring 21 carry male elements 36, shaped like a hammerhead forexample, which engage respective female elements 37, consisting ofrespective seats formed in the outer ring 21; instead, the connectionportions 30 facing towards the inner ring 22 are provided withrespective female elements 37 or seats, which are engaged by respectivemale elements 36, these also, for example, shaped like a hammerhead,carried by the inner ring 22.

The blades 12, including the connection portions 29 and 30, andpreferable also the half-rings 23 and 24, are made by casting with anickel-based metal alloy, for example, of the CX2MW type according toASTM A494, similar to the alloy with the commercial name of HastelloyC22®, or another alloy of similar characteristics.

The metal alloys utilized in accordance with the present invention areessentially nickel-chromium-molybdenum-tungsten-iron alloys, havingnickel as the predominant component, with high chromium content andcontaining significant quantities of molybdenum, together with tungstenand iron.

In particular, the alloy utilized contains nickel in quantities greaterthan approximately 55% by weight and preferably greater thanapproximately 58% by weight, chromium in quantities approximatelybetween 20% and 22.5% by weight, molybdenum in quantities approximatelybetween 12.5% and 14.5% by weight, tungsten in quantities approximatelybetween 2.5% and 3.5% by weight and iron in quantities approximatelybetween 2% and 6% by weight.

With regards to corrosion resistance in a geothermal environment, thesealloys demonstrate a distinctly superior behaviour to those ofconventional steels and, in particular, to AISI 403 type martensiticsteels.

The advantages of the present invention with respect to known solutionsclearly emerge from that which has been explained:

-   -   the fixed blade assembly 20 and specifically the stator blades        12 made in accordance with the invention have significantly        superior corrosion resistance with respect to blades made with        traditional materials,    -   the stator blades 12 can be substituted with reutilization of        the support rings 21 and 22 (or the half-rings 23 and 24), with        clearly evident advantages in terms of cost and simplicity of        maintenance operations,    -   the solution of the invention can also be utilized in steam        turbines originally designed for conventional solutions, with        limited need for operations on the original fixed parts.

Finally, it is clearly understood that changes and variations may bemade to that which is described and illustrated herein without departingfrom the scope of the invention as defined by the attached claims.

1. A highly corrosion-resistant fixed blade assembly (20) for a steam turbine, in particular a geothermal impulse turbine, comprising an array (11) of stator blades (12) substantially arranged in a radial pattern around an axis (A) and supported at respective longitudinally opposite ends (27, 28) by an outer support ring (21) and an inner support ring (22); wherein each blade (12) of the assembly (20) is firmly connected to the rings (21, 22) and is supported by the rings via respective mechanical joints (31, 32) devoid of structural welds, so as to be removable from the rings without breaking the rings.
 2. An assembly according to claim 1, wherein the blades (12) are made of a nickel-based metal alloy.
 3. An assembly according to claim 1, wherein each blade (12) is integrally connected to the rings (21, 22) and is supported by the rings (21, 22) via mechanical joint members (35) arranged at the ends (27, 28) of the blade.
 4. An assembly according to claim 3, wherein each blade (12) is supported by the rings (21, 22) exclusively via the joint members (35).
 5. An assembly according to claim 3, wherein the joint members (35) comprise a pair of male elements (36) that engage with respective female elements (37) having a shape matching the shape of the male elements (36).
 6. An assembly according to claim 5, wherein the male elements (36) are inserted in the respective female elements (37) with clearances that are eliminated by wedges or shims.
 7. An assembly according to claim 5, wherein each blade (12) carries a male element (36) and a female element (37) arranged at the respective ends (27 and 28) of the blade.
 8. An assembly according to claim 1, wherein each blade (12) has two connection portions (29, 30) arranged at the ends (27, 28) of the blade and which project transversally from the ends of the blade.
 9. An assembly according to claim 8, wherein the connection portions (29 and 30) respectively carry a male element (36) that engages a female element (37) formed in a first ring (21), and a female element (37) that is engaged by a male element (36) carried b a second ring (22).
 10. An assembly according to claim 8, wherein the connection portions (29 and 30) are made as an integral part of the blades (12) in the same material as the blades.
 11. An assembly according to claim 1, wherein the blades (12) are made h casting with a nickel-based metal alloy containing chromium, molybdenum, tungsten and iron.
 12. An assembly according to claim 1, wherein the blades (12) are made with a metal alloy of the CX2MW type according, to ASTM A494 or another alloy of similar characteristics.
 13. An assembly according to claim 1, wherein the blades (12) are made by casting with a nickel-based metal alloy of the Hastelloy® family or a similar material.
 14. A steam turbine (1), in particular a geothermal impulse turbine, including a stator (5) and a rotor disc (6) integrally connected to a shaft (7) rotating around an axis (A) of rotation, wherein, the stator (5) of the turbine (1) includes a fixed blade assembly (20) according to claim
 1. 